The present invention is directed to methods for obtaining improved contrast density in migration imaging members. More specifically, the present invention is directed to processes for improving the contrast density of migration imaging members by selective transparentization of migration marking material. One embodiment of the present invention is directed to a process which comprises (a) providing a migration imaging member comprising (1) a substrate and (2) a softenable layer comprising a softenable material and a photosensitive migration marking material present in the softenable layer as a monolayer of particles situated at or near the surface of the softenable layer spaced from the substrate; (b) uniformly charging the imaging member; (c) imagewise exposing the charged imaging member to activating radiation at a wavelength to which the migration marking material is sensitive; (d) subsequent to step (c), causing the softenable material to soften and enabling a first portion of the migration marking material to migrate through the softenable material toward the substrate in an imagewise pattern while a second portion of the migration marking material remains substantially unmigrated within the softenable layer; and (e) contacting the second portion of the migration marking material with a transparentizing agent which transparentizes migration marking material. Optionally, thereafter, the migration imaging member can be uniformly charged and uniformly exposed to activating radiation at a wavelength to which the migration marking material is sensitive, followed by again causing the softenable material to soften and enabling the first portion of the migration marking material to migrate further through the softenable material toward the substrate.
Migration imaging systems capable of producing high quality images of high optical contrast density and high resolution have been developed. Such migration imaging systems are disclosed in, for example, U.S. Pat. Nos. 5,215,838, 5,202,206, 5,102,756, 5,021,308, 4,970,130, 4,937,163, 4,883,731, 4,880,715, 4,853,307, 4,536,458, 4,536,457, 4,496,642, 4,482,622, 4,281,050, 4,252,890, 4,241,156, 4,230,782, 4,157,259, 4,135,926, 4,123,283, 4,102,682, 4,101,321, 4,084,966, 4,081,273, 4,078,923, 4,072,517, 4,065,307, 4,062,680, 4,055,418, 4,040,826, 4,029,502, 4,028,101, 4,014,695, 4,013,462, 4,012,250, 4,009,028, 4,007,042, 3,998,635, 3,985,560, 3,982,939, 3,982,936, 3,979,210, 3,976,483, 3,975,739, 3,975,195 , and 3,909,262, the disclosures of each of which are totally incorporated herein by reference, and in "Migration Imaging Mechanisms, Exploitation, and Future Prospects of Unique Photographic Technologies, XDM and AMEN", P. S. Vincett, G. J. Kovacs, M. C. Tam, A. L. Pundsack, and P. H. Soden, Journal of Imaging Science 30 (4) July/August, pp. 183-191 (1986), the disclosure of which is totally incorporated herein by reference.
The expression "softenable" as used herein is intended to mean any material which can be rendered more permeable, thereby enabling particles to migrate through its bulk. Conventionally, changing the permeability of such material or reducing its resistance to migration of migration marking material is accomplished by dissolving, swelling, melting, or softening, by techniques, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents, and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means.
The expression "fracturable" layer or material as used herein means any layer or material which is capable of breaking up during development, thereby permitting portions of the layer to migrate toward the substrate or to be otherwise removed. The fracturable layer is preferably particulate in the various embodiments of the migration imaging members. Such fracturable layers of marking material are typically contiguous to the surface of the softenable layer spaced apart from the substrate, and such fracturable layers can be substantially or wholly embedded in the softenable layer in various embodiments of the imaging members.
The expression "contiguous" as used herein is intended to mean in actual contact, touching, also, near, though not in contact, and adjoining, and is intended to describe generically the relationship of the fracturable layer of marking material in the softenable layer with the surface of the softenable layer spaced apart from the substrate.
The expression "optically sign-retained" as used herein is intended to mean that the dark (higher optical density) and light (lower optical density) areas of the visible image formed on the migration imaging member correspond to the dark and light areas of the illuminating electromagnetic radiation pattern.
The expression "optically sign-reversed" as used herein is intended to mean that the dark areas of the image formed on the migration imaging member correspond to the light areas of the illuminating electromagnetic radiation pattern and the light areas of the image formed on the migration imaging member correspond to the dark areas of the illuminating electromagnetic radiation pattern.
The expression "optical contrast density" as used herein is intended to mean the difference between maximum optical density (D.sub.max) and minimum optical density (D.sub.min) of an image. Optical density is measured for the purpose of this invention by diffuse densitometers with a blue Wratten No. 94 filter. The expression "optical density" as used herein is intended to mean "transmission optical density" and is represented by the formula: EQU D=log.sub.10 I.sub.o /I!
where I is the transmitted light intensity and I.sub.o is the incident light intensity. For the purpose of this invention, all values of transmission optical density given in this invention include the substrate density of about 0.2 which is the typical density of a metallized polyester substrate.
High optical density in migration imaging members allows high contrast densities in migration images made from the migration imaging members. High contrast density is highly desirable for most information storage systems. Contrast density is used herein to denote the difference between maximum and minimum optical density in a migration image. The maximum optical density value of an imaged migration imaging member is, of course, the same value as the optical density of an unimaged migration imaging member.
There are various other systems for forming such images, wherein non-photosensitive or inert marking materials are arranged in the aforementioned fracturable layers, or dispersed throughout the softenable layer, as described in the aforementioned patents, which also disclose a variety of methods which can be used to form latent images upon migration imaging members.
Various means for developing the latent images can be used for migration imaging systems. These development methods include solvent wash away, solvent vapor softening, heat softening, and combinations of these methods, as well as any other method which changes the resistance of the softenable material to the migration of particulate marking material through the softenable layer to allow imagewise migration of the particles in depth toward the substrate. In the solvent wash away or meniscus development method, the migration marking material in the light struck region migrates toward the substrate through the softenable layer, which is softened and dissolved, and repacks into a more or less monolayer configuration. In migration imaging films supported by transparent substrates alone, this region exhibits a maximum optical density which can be as high as the initial optical density of the unprocessed film. On the other hand, the migration marking material in the unexposed region is substantially washed away and this region exhibits a minimum optical density which is essentially the optical density of the substrate alone. Therefore, the image sense of the developed image is optically sign reversed. Various methods and materials and combinations thereof have previously been used to fix such unfixed migration images. One method is to overcoat the image with a transparent abrasion resistant polymer by solution coating techniques. In the heat or vapor softening developing modes, the migration marking material in the light struck region disperses in the depth of the softenable layer after development and this region exhibits D.sub.min which is typically in the range of 0.6 to 0.7. This relatively high D.sub.min is a direct consequence of the depthwise dispersion of the otherwise unchanged migration marking material. On the other hand, the migration marking material in the unexposed region does not migrate and substantially remains in the original configuration, i.e. a monolayer. In migration imaging films supported by transparent substrates, this region exhibits a maximum optical density (D.sub.max) of about 1.8 to 1.9. Therefore, the image sense of the heat or vapor developed images is optically sign-retained.
Techniques have been devised to permit optically sign-reversed imaging with vapor development, but these techniques are generally complex and require critically controlled processing conditions. An example of such techniques can be found in U.S. Pat. No. 3,795,512, the disclosure of which is totally incorporated herein by reference.
For many imaging applications, it is desirable to produce negative images from a positive original or positive images from a negative original (optically sign-reversing imaging), preferably with low minimum optical density. Although the meniscus or solvent wash away development method produces optically sign-reversed images with low minimum optical density, it entails removal of materials from the migration imaging member, leaving the migration image largely or totally unprotected from abrasion. Although various methods and materials have previously been used to overcoat such unfixed migration images, the post-development overcoating step can be impractically costly and inconvenient for the end users. Additionally, disposal of the effluents washed from the migration imaging member during development can also be very costly.
The background portions of an imaged member can sometimes be transparentized by means of an agglomeration and coalescence effect. In this system, an imaging member comprising a softenable layer containing a fracturable layer of electrically photosensitive migration marking material is imaged in one process mode by electrostatically charging the member, exposing the member to an imagewise pattern of activating electromagnetic radiation, and softening the softenable layer by exposure for a few seconds to a solvent vapor thereby causing a selective migration in depth of the migration material in the softenable layer in the areas which were previously exposed to the activating radiation. The vapor developed image is then subjected to a heating step. Since the exposed particles gain a substantial net charge (typically 85 to 90 percent of the deposited surface charge) as a result of light exposure, they migrate substantially in depth in the softenable layer towards the substrate when exposed to a solvent vapor, thus causing a drastic reduction in optical density. The optical density in this region is typically in the region of 0.7 to 0.9 (including the substrate density of about 0.2) after vapor exposure, compared with an initial value of 1.8 to 1.9 (including the substrate density of about 0.2). In the unexposed region, the surface charge becomes discharged due to vapor exposure. The subsequent heating step causes the unmigrated, uncharged migration material in unexposed areas to agglomerate or flocculate, often accompanied by coalescence of the marking material particles, thereby resulting in a migration image of very low minimum optical density (in the unexposed areas) in the 0.25 to 0.35 range. Thus, the contrast density of the final image is typically in the range of 0.35 to 0.65. Alternatively, the migration image can be formed by heat followed by exposure to solvent vapors and a second heating step which also results in a migration image with very low minimum optical density. In this imaging system as well as in the previously described heat or vapor development techniques, the softenable layer remains substantially intact after development, with the image being self-fixed because the marking material particles are trapped within the softenable layer.
The word "agglomeration" as used herein is defined as the coming together and adhering of previously substantially separate particles, without the loss of identity of the particles.
The word "coalescence" as used herein is defined as the fusing together of such particles into larger units, usually accompanied by a change of shape of the coalesced particles towards a shape of lower energy, such as a sphere.
Generally, the softenable layer of migration imaging members is characterized by sensitivity to abrasion and foreign contaminants. Since a fracturable layer is located at or close to the surface of the softenable layer, abrasion can readily remove some of the fracturable layer during either manufacturing or use of the imaging member and adversely affect the final image. Foreign contamination such as finger prints can also cause defects to appear in any final image. Moreover, the softenable layer tends to cause blocking of migration imaging members when multiple members are stacked or when the migration imaging material is wound into rolls for storage or transportation. Blocking is the adhesion of adjacent objects to each other. Blocking usually results in damage to the objects when they are separated.
The sensitivity to abrasion and foreign contaminants can be reduced by forming an overcoating such as the overcoatings described in U.S. Pat. No. 3,909,262, the disclosure of which is totally incorporated herein by reference. However, because the migration imaging mechanisms for each development method are different and because they depend critically on the electrical properties of the surface of the softenable layer and on the complex interplay of the various electrical processes involving charge injection from the surface, charge transport through the softenable layer, charge capture by the photosensitive particles and charge ejection from the photosensitive particles, and the like, application of an overcoat to the softenable layer can cause changes in the delicate balance of these processes and result in degraded photographic characteristics compared with the non-overcoated migration imaging member. Notably, the photographic contrast density can degraded. Recently, improvements in migration imaging members and processes for forming images on these migration imaging members have been achieved. These improved migration imaging members and processes are described in U.S. Pat. No. 4,536,458 and U.S. Pat. No. 4,536,457.
U.S. Pat. No. 4,937,163 (Tam et al.), the disclosure of which is totally incorporated herein by reference, discloses an imaging member which comprises an ionically conductive film forming polymer, such as sulfonated polystyrene, and an electrically insulating softenable layer comprising a fracturable layer containing electrically photosensitive migration marking particles.
U.S. Pat. No. 4,880,715 (Tam et al.), the disclosure of which is totally incorporated herein by reference, discloses an imaging system in which an imaging member comprising a substrate and an electrically insulating softenable layer on the substrate, the softenable layer comprising migration marking material located at least at or near the surface of the softenable layer spaced from the substrate, and a charge transport material in the softenable layer is imaged by eletrostatically charging the member, exposing the member to activating radiation in an imagewise pattern, decreasing the resistance to migration of marking material in the softenable layer sufficiently to allow the migration marking material struck by said activating radiation to retain a slight net charge which allows only slight agglomeration, slight coalescence, slight migration in depth of marking material towards said substrate or combination thereof in image configuration during a further decreasing of the resistance to migration towards the substrate in image configuration, and further decreasing the resistance to migration of marking material in the softenable layer sufficiently to allow non-exposed marking material to agglomerate and coalesce substantially. This imaged member may be used as a xeroprinting master in a xeroprinting process comprising uniformly charging the master, uniformly exposing the charged master to activating illumination to form an electrostatic latent image, developing the latent image to form a toner image and transferring the toner image to a receiving member. A charge transport spacing layer comprising a film forming binder and a charge transport compound may be employed between the substrate and the softenable layer in order to increase the surface potential associated with the surface charges of the latent image.
U.S. Pat. No. 4,853,307 (Tam et al.), the disclosure of which is totally incorporated herein by reference, discloses an imaging system including a migration imaging member comprising a substrate and an electrically insulating softening layer adjacent the substrate, the softenable layer comprising a fracturable layer of electrically photosensitive migration marking material located substantially at or near the surface of the softenable layer spaced from the substrate, and a copolymer of styrene and ethyl acrylate in at least one layer adjacent the substrate, the copolymer comprising between about 40 and about 80 mole percent styrene, between about 20 and about 60 mole percent ethyl acrylate, the copolymer having a M.sub.n between about 4,000 and about 35,000, a M.sub.w between about 10,000 and about 80,000, a T.sub.g between about 30.degree. C. and about 75.degree. C., and a melt viscosity between about 1.times.10.sup.2 poise and about .times.10.sup.6 poise at 115.degree. C. The migration imaging member may be imaged by charging, imagewise exposing to activating radiation and developing with heat, solvent vapor, or solvent vapor pretreatment followed by heat. Some embodiments of the imaged member, wherein the softenable layer contains a charge transport material, may be utilized as a master in a xeroprinting process. The copolymer of styrene and ethyl acrylate may be in an adhesive layer of charge transport spacing layer between the substrate and the softenable layer, or in the softenable layer itself. If desired, the copolymer in any of the aforesaid layers may be a terpolymer of styrene, ethyl acrylate and a copolymerizable organic acid having carbon-to-carbon unsaturation or a copolymerizable derivative of the organic acid.
U.S. Pat. No. 4,536,457 (Tam), the disclosure of which is totally incorporated herein by reference, discloses an imaging method comprising providing a migration imaging member comprising a substrate and an electrically insulating softenable layer on the substrate, the softenable layer comprising migration marking material located at least at or near the surface of the softenable layer spaced from the substrate and a charge transport material in the softenable layer, electrostatically charging the member, exposing the member to activating radiation in an imagewise pattern, decreasing the resistance to migration of marking material in the softenable layer sufficiently to allow slight migration in depth of marking material towards the substrate in image configuration, and further decreasing the resistance to migration of marking material in the softenable layer sufficiently to allow nonmigrated marking material to agglomerate.
U.S. Pat. No. 4,252,890 (Haas et al.), the disclosure of which is totally incorporated herein by reference, discloses an imaging system wherein a migration-type imaging member comprising a softenable layer containing agglomerable migration marking material is provided, and the member is exposed to an image pattern of electromagnetic radiation of sufficient energy to cause a simultaneous imagewise migration at least in depth in the softenable layer and agglomeration of the agglomerable migration marking material in the imagewise exposed areas of the imaging member. In another embodiment, a microscopically discontinuous layer of imaging material on a stable substrate is agglomeration or evaporation imaged by the inventive system.
U.S. Pat. No. 4,241,156 (Haas et al.), the disclosure of which is totally incorporated herein by reference, discloses an imaging system wherein a migration-type imaging member comprising a softenable layer containing agglomerable migration marking material is provided, and the member is exposed to an image pattern of electromagnetic radiation of sufficient energy to cause a simultaneous imagewise migration at least in depth in the softenable layer and agglomeration of the agglomerable migration marking material in the imagewise exposed areas of the imaging member. In another embodiment, a microscopically discontinuous layer of imaging material in a stable substrate is agglomeration or evaporation imaged by the inventive system.
U.S. Pat. No. 4,101,321 (Levy et al.), the disclosure of which is totally incorporated herein by reference, discloses an imaging system wherein an imaged migration-type imaging member is provided and either the background of image areas of said image are selectively reduced to a more transparent condition. The imaged member comprises a softenable layer containing agglomerable materials in both image and complementary background configurations. This member is contacted with solvent vapors capable of softening the softenable layer and heated, thereby causing the agglomerable material to selectively agglomerate in one of either the background or image areas.
U.S. Pat. No. 4,084,966 (Haas et al.), the disclosure of which is totally incorporated herein by reference, discloses an imaging system wherein a migration-type imaging member comprising a softenable layer containing agglomerable migration marking material is provided, and the member is exposed to an image pattern of electromagnetic radiation of sufficient energy to cause a simultaneous imagewise migration at least in depth in the softenable layer and agglomeration of the agglomerable migration marking material in the imagewise exposed areas of the imaging member. In another embodiment, a microscopically discontinuous layer of imaging material on a stable substrate is agglomeration or evaporation imaged by the inventive system.
U.S. Pat. No. 4,065,307 (Goffe), the disclosure of which is totally incorporated herein by reference, discloses an imaging system comprising providing an imaging member comprising an agglomerable layer in contact with an imagewise hardenable-softenable layer and imagewise hardening said member. An image is developed by imagewise softening said member to cause relative transparentizing in the imagewise softened areas due to an agglomeration of the agglomerable layer in the imagewise softened portions of said member.
U.S. Pat. No. 4,029,502 (Goffe), the disclosure of which is totally incorporated herein by reference, discloses an imaging system comprising providing an imaging member comprising an agglomerable layer contacting a softenable layer and imagewise softening said member to cause relative transparentizing of said member in softened areas due to an agglomeration of the agglomerable layer in softened portions of said member.
U.S. Pat. No. 5,108,861 (Kovacs et al.), the disclosure of which is totally incorporated herein by reference, discloses an electrostatographic device which includes a metal halide conductive transparent layer which is free of nonuniformities. Very thin layers of metal halides are formed for imaging members by vacuum evaporation and exhibit adequate conductivity and transparency.
Migration imaging members are also suitable for use as masks for exposing the photosensitive material in a printing plate. The migration imaging member can be laid on the plate prior to exposure to radiation, or the migration imaging member layers can be coated or laminated onto the printing plate itself prior to exposure to radiation, and removed subsequent to exposure.
U.S. Pat. No. 5,102,756 (Vincett et al.), the disclosure of which is totally incorporated herein by reference, discloses a printing plate precursor which comprises a base layer, a layer of photohardenable material, and a layer of softenable material containing photosensitive migration marking material. Alternatively, the precursor can comprise a base layer and a layer of softenable photohardenable material containing photosensitive migration marking material. Also disclosed are processes for preparing printing plates from the disclosed precursors.
Copending application U.S. Ser. No. 08/353,461, filed Dec. 9, 1994, entitled "Improved Migration Imaging Members," with the named inventors Edward G. Zwartz, Carol A. Jennings, Man C. Tam, Philip H. Soden, Arthur Y. Jones, Arnold L. Pundsack, Enrique Levy, Ah-Mee Hor, and William W. Limburg, the disclosure of which is totally incorporated herein by reference, discloses a migration imaging member comprising a substrate, a first softenable layer comprising a first softenable material and a first migration marking material contained at or near the surface of the first softenable layer spaced from the substrate, and a second softenable layer comprising a second softenable material and a second migration marking material. Also disclosed is a migration imaging process employing the aforesaid imaging member.
Copending application U.S. Ser. No. 08/413,667, filed Mar. 30, 1995, entitled "Improved Apparatus and Process for Preparation of Migration Imaging Members," with the named inventors Philip H. Soden and Arnold L. Pundsack, the disclosure of which is totally incorporated herein by reference, discloses an apparatus for evaporation of a vacuum evaporatable material onto a substrate, said apparatus comprising (a) a walled container for the vacuum evaporatable material having a plurality of apertures in a surface thereof, said apertures being configured so that the vacuum evaporatable material is uniformly deposited onto the substrate; and (b) a source of heat sufficient to effect evaporation of the vacuum evaporatable material from the container through the apertures onto the substrate, wherein the surface of the container having the plurality of apertures therein is maintained at a temperature equal to or greater than the temperature of the vacuum evaporatable material.
Copending application U.S. Ser. No. 442,227, filed concurrently herewith, entitled "Improved Migration Imaging Members," with the named inventors Shadi L. Malhotra, Liquin Chen, and Marie-Eve Perron, the disclosure of which is totally incorporated herein by reference, discloses a migration imaging member comprising (a) a substrate, (b) a softenable layer comprising a softenable material and a photosensitive migration marking material, and (c) a transparentizing agent which transparentizes migration marking material in contact therewith contained in at least one layer of the migration imaging member. Also disclosed is a process which comprises (1) providing a migration imaging member comprising (a) a substrate, (b) a softenable layer comprising a softenable material and a photosensitive migration marking material, and (c) a transparentizing agent which transparentizes migration marking material in contact therewith contained in at least one layer of the migration imaging member; (2) uniformly charging the imaging member; (3) subsequent to step (2), exposing the charged imaging member to activating radiation at a wavelength to which the migration marking material is sensitive; (4) subsequent to step (3), causing the softenable material to soften and enabling a first portion of the migration marking material to migrate through the softenable material toward the substrate in an imagewise pattern while a second portion of the migration marking material remains substantially unmigrated within the softenable layer, wherein subsequent to migration of the first portion of migration marking material, either (a) the first portion of migration marking material contacts the transparentizing agent and the second portion of migration marking material does not contact the transparentizing agent; or (b) the second portion of migration marking material contacts the transparentizing agent and the first portion of migration marking material does not contact the transparentizing agent.
While known apparatus and processes are suitable for their intended purposes, a need remains for methods of improving the optical contrast density of migration imaging members. Further, there is a need for methods for improving optical contrast density of migration imaging members wherein the optical density of the D.sub.min areas of the imaged member is decreased without a corresponding decrease in the optical density of the D.sub.max areas of the imaged member. Additionally, there is a need for methods for improving optical contrast density of migration imaging members wherein the optical density of the D.sub.min areas of the imaged member with respect to ultraviolet light passing through the imaging member is decreased without a corresponding decrease in the optical density of the D.sub.max areas of the imaged member with respect to ultraviolet light passing through the imaging member. There is also a need for methods of developing migration imaging members wherein the image is sign-reversed.